Some conventional control apparatuses for controlling driving of a three-phase rotary machine include two power supply systems of power converters in correspondence to two three-phase coil sets, each of which includes three phase coils of a three-phase rotary machine.
According to JP 02-70286A (patent document 1) corresponding to U.S. Pat. No. 4,814,964, for example, an AC motor drive apparatus is provided with two three-phase inverters for a motor, which has two coil sets angularly spaced apart by an angle of 30° from each other. Each coil set includes three phase coils. The two three-phase inverters are connected to the two coil sets of an AC motor, respectively. The two three-phase inverters generate three-phase AC currents, which are phase-shifted, from a DC pulse current. The inverters supply the AC currents to the two three-phase coil sets. Thus torque ripple (fluctuation of torque) of the motor is reduced.
It is necessary to prevent a rotary machine such as a motor from damages caused by heat. JP 4356295 (patent document 2) proposes to protect an electric power steering system, which assists steering of a vehicle, from overheating of a motor by estimating a motor temperature based on an integrated value of a motor current and limiting the motor current so that the estimated motor current does not exceed a predetermined temperature.
The overheat prevention according to the patent document 2 is provided for a system, which has one coil set and one power converter. If this overheat prevention is applied to a motor of two power supply systems, which includes two coil sets and two power converters for a motor as exemplified in patent document 1, it is likely that the overheat protection is applied to each of two power supply systems. In this case, when the estimated temperature rises in one of the two power supply systems, the current will be limited only in one power supply system actually suffering from overheat. As a result, the currents supplied in the two power supply systems differ and increase torque ripples. Although shifting of phase of currents supplied by the two power supply systems is effective to suppress torque ripple, such shifting does not contribute to suppression of torque ripple. In case that the motor and the control apparatus are applied to an electric power steering system, the increased torque ripple will adversely influence on a driver's steering operation.